One example of a manufacturing process that requires controlled evacuation and repressurization of a work station is the process of controlled doping of silicon wafers with ions in an ion implantation chamber. Ions from a source are accelerated along an ion travel path to impinge upon the wafers and introduce controlled doses of impurities into the silicon wafer. The ion travel path must be evacuated to assure the ions are well collimated. To accomplish this process in the prior art, wafers have been introduced to an ion implantation chamber, either through a loadlock or by introducing the wafers directly into the implantation chamber. If a loadlock arrangement is used; the loadlock chamber is successively evacuated and pressurized as wafers are inserted into the loadlock on their travel path to the ion implantation chamber. If no loadlock is used, the wafers are inserted directly into the ion implantation chamber which itself must be pressurized, evacuated, and then repressurized as the workpiece are inserted into the chamber, treated and then removed.
Other examples of processes involving pressurization and depressurization of a chamber are known in the prior art. In a sputter coating procedure, for example, a workpiece is inserted into a treatment chamber and then a coating is applied to the surface of the work piece by sputtering the coating material away from a target. This procedure can be used, for example, in coating magnetic material into a recording medium. Again, prior to conducting the coating process, the work piece must be inserted into the chamber and then the coating process conducted at a reduced pressure.
It is often a requirement in these processes that the level of contaminants within the processing chamber is kept at a minimum. If the contaminant level in a doping chamber, for example, exceeds a specified value, the semi-conductor yield of the process will be reduced.
Although steps are taken to reduce the level of particulate contamination within a processing chamber, these steps cannot totally avoid such contaminants. Particulates are inevitably introduced, for example, as the workpieces are inserted into an ion implantation chamber. These particulates tend to settle on the interior walls of the chamber and remain in place until air flow that occurs during chamber evacuation and repressurization dislodge the particles causing them to move within the chamber. If the particulate contaminants remain attached to the chamber walls, the work piece can be inserted into the chamber, treated, and removed without undue contamination. It is when the particles are dislodged and come to rest on the workpiece either before or after the ion implantation process that the particles' presence reduces production yield.
Prior art U.S. Pat. No. 4,739,787 to Stoltenberg which issued Apr. 26, 1988 recognizes the possibility for contaminant presence effecting the yields during semi-conductor wafer fabrication. This patent recognizes the possibility of dislodging contaminants from chamber walls as air enters and exits the process chamber. As a proposed solution to the dislodging problem, the '787 patent recommends the use of "soft-start valves" which open in accordance with a controlled profile so that "turn on" turbulence is reduced. Specifically, the '787 patent calls for a pressurization and depressurization of a chamber in accordance with a time profile.
Prior art loadlocks have at least partially addressed particle transport mechanisms, but none has been able to achieve evacuation times of the order of two seconds to satisfactory throughput in an ion implanter which processes wafers serially.
Recent attention has been focused on aerodynamic transport of particles under supersonic flow conditions. See U.S. Pat. No. 4,836,233 to Milgate, Ill. See also U.S. Pat. No. 4,987,933 to Mack, which is incorporated herein by reference.
During venting of a loadlock, if supersonic flow occurs within the system, it can produce turbulence that strips particles from the surfaces in fluid communication with the loadlock. The Mack design isolates the region of supersonic flow from particle contamination by confining the supersonic flow to a region in the annular vent gas inlet region by using large flow expansion in this region so the flow transitions to subsonic, and by assuring that under all operating conditions, flow is out of this region into the lock bell to prevent influx of particles. These features work together to minimize the deleterious effects of supersonic flow.
Turbulence presents several problems in areas where particle transport is to be minimized. In laminar flow, the lines of flow tend to transport particles in the direction of bulk flow, but in turbulent flow in which eddy currents occur throughout the flow stream, particles can flow upstream of the bulk flow direction. In corners and dead spaces where there are abrupt changes in flow area, eddy currents cause flow to recirculate under either laminar or turbulent flow. These eddy currents and recirculating flows interfere with any efforts to clean the chamber using the flow of fluid over the internal surfaces. Laminar flow is thus to be preferred for its superior ability to sweep already entrained particles along with the bulk flow--a property essential for effective flow induced cleaning. Smooth, well-controlled flow sections are also desirable.
A second problem with turbulent flow is that it is characterized by a higher surface stress and thus more effectively removes and entrains particles to chamber surfaces. This property, while being valuable during cleaning, is problematic during venting since it will tend to detach particles from the wall and deposit them on the wafer surface.
It is thus important to take whatever measures can be taken to eliminate the possibility of turbulent flow and to eliminate regions of abrupt changes in flow area which might trap particles during cleaning.
The prior art system depicted in FIGS. 14A, 14B includes a chamber C with fluid ports P extending circumferentially around a wall of the chamber and located beneath a wafer W supported within the chamber C. Flow paths within the chamber are non-radial. This design is incorporated in a medium current implanter available from Eaton under the designations 6200 A and AV. With non-radial flow, the maximum flow velocity is not minimized. For example, assume that there is one hole at the chamber edge for admitting vent gases and the maximum flow velocity occurs at that hole. If there were two holes, the maximum velocity could be reduced by half. Addition of holes at the periphery would further decrease the maximum velocity until in the limit there is a slit of given width around the periphery of the chamber. This line of reasoning demonstrates that they way to minimize the peak flow velocity is to introduce air around the periphery as evenly as possible. This causes the Reynolds number to be minimized and thus the viscous drag and the probability of entraining particles. Another disadvantage of this lock is that flow can sweep particles from the back of the wafer or the bottom of the loadlock onto the font face of the wafer.
A prior art medium current implanter from Varian, designated by Model No. E220, is believed to include a chamber with large dimensions transverse to the flow path and abrupt changes in the flow cross-sectional area. The fluid flow in this system is believed to be generally radial around the wafers during both pumping and venting. The abrupt changes in flow cross section, however, may cause eddy currents which make flow cleaning difficult. The large dimensions transverse to flow cause large Reynolds numbers and thus decrease the velocity at which turbulent flow and thus particle entrainment occur. The large dimension also should result in longer pumping time. There is also evidence that the higher the Volume to Surface ratio, the more particle agglomeration occurs at a given level of humidity.
Surface preparation has been shown to affect particle generation and transport in high purity gas distribution systems. It has also been shown to affect outgassing characteristics of materials. Treatments which create smooth, continuous, dense and inert surfaces are advantageous in both respects. These surfaces can be achieved by combinations of mechanical, electro-chemical and chemical treatments which may include polishing and cleaning processes. Application of these surface treatment techniques can improve the lock's initial particle performance and thus improve its transport of particles at initial pumpdown since lower gas loads from outgassing either shorten the pumping time or allow more restrictive throttling of gas flow during early parts of the evacuation cycle.